1. Field of the Invention
The invention generally relates to a structure of a metal gate and a fabrication method thereof. More particularly, the invention relates to a structure and a fabrication method of a gate-last metal gate.
2. Description of the Prior Art
With a trend towards scaling down the size of semiconductor devices, conventional methods used to achieve optimization, such as reducing thickness of the gate dielectric layer (for example, the thickness of a silicon dioxide layer) have faced problems such as leakage current due to tunneling effect. In order to continue the progression towards the next generation, high-K materials are used to replace the conventional silicon oxide to be the gate dielectric layer because they can decrease physical limit thickness effectively, reduce leakage current, and obtain an equivalent capacitance in an identical equivalent oxide thickness (EOT).
A conventional polysilicon gate also has certain disadvantages, such as inferior performance due to boron penetration, and an unavoidable depletion effect, which increases the equivalent thickness of the gate dielectric layer, reduces gate capacitance, and worsens a driving force of devices. Thus, work function metals have been developed to replace the conventional polysilicon gate to be the control electrode that is competent to the high-K gate dielectric layer.
There is always a continuing need in the semiconductor processing art to develop semiconductor devices which have a superior performance and reliability even though the conventional silicon dioxide or silicon oxynitride gate dielectric layer is replaced by a high-K gate dielectric layer and the conventional polysilicon gate is replaced by a metal gate.